The present invention is a method and apparatus that pertain to printing systems. More particularly, this invention provides a capillary wave printer that accurately delivers a high density, variable intensity pattern of ink droplets onto a projection surface at very high speeds.
A printer is a device which transfers information, either graphics or text, from a computer medium to hardcopy, such as paper. The speed at which the paper hardcopy may be produced, the clarity and the resolution of the hardcopy are measures of the quality of the printer. Resolution is a measure of the capability of a printer to reproduce fine detail on paper. The higher the resolution of the printer, the more faithful the reproduction of the original text or graphics and the more impressive the final product. The technology utilized determines the quality of the printer and its ultimate cost.
The use of capillary surface waves (i.e., those waves which travel on the surface of a liquid in a regime where the surface tension of the liquid is such a dominating factor that gravitational forces have negligible effect on the wave behavior) are attractive for liquid ink printing and are known to persons ordinarily skilled in the electronic printing arts (e.g. U.S. Pat. No. 4,719,476 entitled "Spatially Addressing Capillary Wave Droplet Ejectors and the Like", and U.S. Pat. No. 4,719,480 entitled "Spatial Stabilization of Standing Capillary Surface Waves"). The spatial frequency range in which capillary waves exist spans and extends well beyond the range of resolutions within which non-impact printers normally operate. The method of selectively addressing individual wave crests of standing capillary surface waves to eject droplets from the selected crests on command is well-known to persons ordinarily skilled in the electronic printing arts. To this end, the addressing mechanims locally alter the surface properties of the selected wave crests, such as the local surface pressure acting on the selected crests and/or the local surface tension of the liquid within the selected crests. Discrete addressing mechanisms are especially attractive for printing, not only because their individual addressing elements may be spatially fixed with respect to one dimension of the recording medium, but also because the spatial frequency of their addressing elements may be matched to the spatial frequency of the capillary wave. Such spatial frequency matching enables selected crests of the capillary wave to be addressed in parallel, thereby allowing droplets to be ejected in a controlled manner from the selected crests substantially simultaneously, such as for line printing.
The problem of printing high resolution graphical images very quickly and with faithful gray-scale rendition has presented a major challenge to the printer industry. The transfer of high resolution black and white graphic images from a computer screen to hardcopy requires that each picture element, or pixel, in the computer memory be faithfully reproduced in true relative intensity. True relative intensity is expressed in shades of gray which is a continuous scale of brightness between a minimum black level and a maximum white level. Most printers produce "half-tone" images rather than true shades of gray. A half-tone image is a spatial arrangement of black and white "dots" which creates a graphics image on a computer screen or on paper. Half-tone images are easy to create since only black and white "dots" are required, however, the resultant image is lacking in resolution and clarity when compared to a black and white photograph for example.
Since many computers now have video graphics capability, whereby each pixel on the screen is assigned its own unique shade of gray falling somewhere between black and white, full resolution photographic quality images are available to the computer user. Printing a hardcopy of what is seen on the computer display (which contains multiple shades of gray) is however not possible with today's binary pixel printers (i.e., black and white, no gray-scale). A compromise approach which falls short of true multi-level gray-scale printing, but which offers image quality superior to that of fixed spot size half-tone printing, is the variable spot size, pseudo-gray-scale printing technique.
If a printer were able to effectively produce high quality, pseudo-gray-scale images at an affordable price, then such a printer would be in very high demand by consumers. The development of a straightforward method and apparatus which would provide the capability to print high resolution images with pseudo-gray-scale or full color at significantly faster rates would represent a major technological advance to the printer industry.
Though capillary wave droplet ejectors are known in the prior art, no printers utilizing this technology exist on the market today. One of the reasons for this is the lack of a cost effective line printer head which provides for the selective addressing of capillary wave peaks under computer control. Some prior devices utilize an array of discrete addressing electrodes which may be pulsed with short pulses of moderately high voltage electrical energy (coherent with the frequency of the capillary wave) to permit the parallel addressing of selected wave crests. Other earlier devices employ a print head utilizing discrete electrical or thermal addressing elements supported on a suitable substrate, such as a Mylar film, and mounted in a transverse orientation slightly below the free surface of the ink. All current apparatus for selectively addressing capillary wave peaks have shortcomings making them impractical for the marketplace. First of all, thermal addressing mechanisms have too long a time constant, thus limiting the maximum throughput rate achievable. Laser addressing mechanisms require precise opto-mechanical alignments which are complex and costly. And finally, the selective generation of E-fields to destabilize capillary wave peaks creates electro-chemical interactions with the ink supply. Clearly, a highly producible discrete addressing mechanism for a capillary wave printer head which surpasses the current state-of-the-art would enable the variable spot size capillary wave printing technology to advance in the marketplace. The enhanced performance and low-cost print heads which could be produced using such innovative technology would satisfy a long felt need within the printing industry.